106 Dr. F. H. Hatch— Gem- Gravel, W. Coast of Africa. 



III. — Description of a Diamantifekou.s Gem-Gkavel from the 



West Coast of Africa. 



By F. H. Hatch, Ph.D., M.Inst. C.E., Vice-President of the Institution of 



Mining and Metallurgy. 

 rPHE material described in this ]iaper was handed to me by the 

 I Liberian Development Company (to which I acted for some time 

 as technical adviser) and was obtained in the course of prospecting 

 operations prosecuted by that Company at Banja Ta (Montserrado) 

 on the Jiblong and Bor Bivers (tributaries of the Junk Biver), some 

 thirty miles inland from Monrovia, the chief port of the State of Liberia. 



Diamonds and gold were discovered in the alluvial gravels of these 

 rivers by the late Mr. George G. Dixon, and the concentrates submitted 

 to me for examination were brought to this country by Mr. Dixon at 

 the end of 1910. Mr. Dixon returned to Liberia soon after I had 

 seen him ; but unhappily he succumbed to an attack of malarial fever 

 early in 1911. The subsequent prospecting work was carried on 

 under the direction of his assistant, Mr. S. M. Owen, who at the end 

 of the working season of 1911 brought me a series of rock-specimens 

 and cores from boreholes (most of which were put down near the 

 rapids to be referred to presently), so that I am enabled also to give 

 ii brief description of the solid geology of the district. 



The bed-rock on which the alluvial gravels rest consists of an 

 ancient floor of crystalline rocks, comprising: quartz-felspar-biotite 

 gneiss with accessory sphene and zircon; garnet-hypersthene-biotite 

 gneiss, composed of abundant red garnet, strongly pleochroic hyper- 

 sthene, brown mica, chlorite, together with granulitic quartz and 

 a small quantity of striated felspar; and coarsely foliated hornblende 

 schist, consisting of green hornblende, plagioclase, quartz and sphene. 

 All these rocks present a well-markeil banding, the general strike of 

 the country as reported by Mr. Dixon being N.E. and S.W., with a dip 

 to the S.E.,' and give the impression of belonging to an ancient 

 sedimentary series which has been profoundly modified by meta- 

 raorphism. Associated with them are dark-coloured basic intrusions 

 which, as the microscope shows, consist of water-clear plagioclase, 

 green hornblende and hypersthene with marked granulitic structure, 

 constituting a rock of the pyroxene-granulite or norite type. 



Except in the bed of the Biver Jiblong itself, where comparatively 

 unweathered specimens of the rocks can be obtained from the bars 

 that stem the flow of the water and thus give rise to falls and rapids, 

 all the rocks are entirely replaced at the surface by red and yellow 

 lateritic or whitish kaolinized products ; but excellent material for 

 microscopic examination was obtained in the shape of cores from the 

 diamond drill which was freely used in the prospecting operations of 

 the Company. The object of the prospecting was two-fold: (1) to 

 ascertain the quantity and value of the diamantiferous gravel, and 

 (2) to find if possible the source from which the diamonds came. 

 It was with the latter object that the borings were made. To ascertain 

 the extent and value of the diamantiferous gravel the following method 

 of prospecting was adopted : First, sites for pits were marked out at 



^ Cf. J. Parkinson, "A Note on the Petrology and Physiography of Western 

 Liberia" : Quart. Journ. Geol. Soc, vol. liv, 1908, p. 313. 



